2021ApJ...916...95I

We report the systematic analysis of knots, hotspots, and lobes in 57 active galactic nuclei (AGNs) to investigate the variation of the magnetic field along the jet from the sub-parsec base to the terminus on kiloparsec-to-megaparsec scales. Expanding the number of radio/X-ray samples in the work of Kataoka & Stawarz, we analyzed the data in 12 FR I and 30 FR II radio galaxies, 12 quasars, and three BL Lac objects, which contained 76 knots, 42 hotspots, and 29 radio lobes. We first derived the equipartition magnetic fields in the cores and then estimated those in various jet components by assuming B_est ∝ d^–1, where d is the distance from the jet base. On the other hand, the magnetic field in large-scale jets (knots, hotspots, and lobes), B_eq, can be estimated from the observed flux and spatial extent under the equipartition hypothesis. We show that the magnetic field decreases as the distance along the jet increases, but generally in a more gentle way than ∝d^–1. The increase in B_eq/B_est at large d may suggest the deceleration of the jet downstream, but there is no difference between FR I and FR II jets. Moreover, the magnetic fields in the hotspots are systematically larger than those in knots and lobes. Finally, we applied the same analysis to knots and lobes in Centaurus A to check whether the above discussion will hold even in a single jet source.